Drive system for continuous casting plants

ABSTRACT

A drive system for drawing a strand-shaped casting out of the mold of a continuous casting plant comprises drive rollers engageable with the strand and mechanically driven by an electrohydraulic stepping motor which is controlled by electrical pulses from a programmable monitor so that the casting is drawn in accordance with a predetermined operating sequence, preferably such that after a given drawing travel the drive rollers remain at rest or are reversely driven in accordance with the cooling and contraction of the previously drawn strand of material, whereafter the drawing and contraction cycle is repeated.

United States Patent Krall et al. [4 1 June 13, 1972 [54] DRIVE SYSTEM FOR CONTINUOUS 3,290,734 12/1966 CASTING PLANTS 3 l2/l968 3,438,426 4/1969 [72] Inventors: Heribert Krall, Wurzburg; Helmut Mug, 3,478,808 11/1969 Hochberg; Otto Herrmann, Wurzburg; 3,504,732 4/1970 Rudolf Posl, Nurnberg, all of Germany 731 Assignees Siemens Aktiengesellschaft, Berlin & OTHER PUBUCATIONS Munich,Germany Control Engineering, Vol. 9, No. 1, January 1964. TJ2l2.C6. 221 Filed: Sept. 18, 1969 [21] Appl. No.: 858,982 Primary Examiner-R Spencer Annear Attorney-Curt M. Avery, Arthur E. Wilfond, Herbert L. 30 Foreign Application Priority Data Lemerand Dame] Sept 21, 1968 Germany ..P 17 83 032.9 [57] ABSTRACT I 52] U 5 Cl 164 1 64/83 1 64/1 57 A drive system for drawing a strand-shaped casting out of the 164/260 1 64/282 mold of a continuous casting plant comprises drive rollers en- [51] Int Cl B22! 11/00 B22d 27/08 gage-able with the strand and mechanically driven by an elec- 58] Fieid 82 154 5 260 282 trohydraulic stepping motor which is controlled by electrical pulses from a programmable monitor so that the casting is drawn in accordance with a predetermined operating 56] References Cited sequence, preferably such that after a given drawing travel the drive rollers remain at rest or are reversely driven in ac- UNITED STATES PATENTS cordance with the cooling and contraction of the previously 1 088 17] 2/1914 P h 4/260 drawn strand of material, whereafter the drawing and contrace rson one deis re ated 2 l35,l83 11/1938 Junghans ..l64/83 y pe 2.815.551 12/1957 Hessenberg et a] 164/83 6 Claims, 5 Drawing Figures ELECTROHYDRAULIC ELECTRON 1c STEPPING MOTOR CONTROLLER PATENTEDJUH 13 1972 3.669176 sum 10F I I I-I 0 ELECTROHYDRAULIC ELECTRONIC STEPPING MOTOR 'CONTROLLER Fig.1

TIME

TRAVEL PATENTEDJuu 1 3 I972 3.669.176

SHEET 20F 2 l I INPUT I READER I I COUNTER BISTABLE FLIP-FLOP 1". N58 F TIMER I E I'd Lu T 1 CONTROL PULSELMV T I TRANSMlTTER M I J 'STEPPING 5 MOTOR Fig.3

1 {MOLD Q I 5 FREQUENCY 5 DIVIDERS 10g TRANSMISSION B 6 {MOTOR 7 5 L 3 J TORQUE CONVERTER/ CONTROLLER Fig. 4 Fig.5

DRIVE SYSTEM FOR CONTINUOUS CASTING PLANTS Our invention relates to continuous casting plants and more particularly to a drive system for drawing a continuous strandshaped casting out of the mold or molten mass of the casting plant with the aid of driven nip rollers which engage the casting at a locality where the casing is solidified and rigid.

It is known to operate the drive roller in such a casting plant with the aid of a hydraulic motor (German Pat. No. 1,151,354). Also known is to drive the rollers with the aid of an electric motor which may be connected with the drive rollers through clutches or operate in conjunction with brakes (Gennan Pat. No. 977,028). In the known electrical systems the relatively long time constant of the drives does not afford performing very fast pulling strokes, and it is not feasible to accurately adjust the travel of the strand casting or to stop the travel at a very accurately defined moment. Substantially the same disadvantages apply to the use of the hydraulic motors operating with conventional control means.

It is an object of our invention to obviate such deficiencies.

Another object of the invention is to provide strand casting plants with a combination of an electrical and hydraulic drive that affords a rapid strand drawing or pulling travel as well as an accurately controllable speed, distance or stopping point of such travel.

According to our invention we drive the nip rollers of a continuous casting plant by means of an electrohydraulic stepping motor which receives electrical control pulses from a programmable control or monitor device. An electrohydraulic stepping motor in which the electrical stepper and the hydraulic motor proper are combined in a structural unit is known as such, for example, from the periodical Control Engineering January, 1962, pages 73 to 75. With such stepping drive, the angle of rotation or the rotary speed of the output shaft is controlled at high accuracy. The input control signals are pulses of very low electrical voltage or current magnitudes; but the output torque is comparatively large by virtue of the hydraulic components. This is due to the fact that the electrohydraulic stepping motor has inherently a high amplifying gain by virtue of an internal feedback and constitutes a type of servomechanism or digital-analog converter.

The electrical stepping motor (stepper) of such an electrohydraulic unit is a small motor which is switched step-bystep in response to respective electrical pulses. Its angle of rotation is proportional to the number of the input pulses, and its speed of rotation is proportional to the pulse frequency. Details of such an electric stepping motor are described, for example, in US. Pat. No. 3,293,060. The electric stepping motor drives a control valve constituted by a slider and a surrounding sleeve with respective hydraulic ducts. The supply of pressure medium to the hydraulic motor of the unit is controlled in proportion to the relative displacement between sleeve and slider of the valve. Details of this type of control valve are described in US. Pat. Nos. 3,310,284 and 3,079,899. The shaft of the hydraulic motor is coupled with the sleeve of the control valve whose slider is axially displaced by the electrical stepping motor. This provides the abovementioned internal feedback and has the effect that the hydraulic motor follows at high accuracy the movements performed by the electrical stepping motor. The electrical pulses for the stepping motor are furnished from a control device for the type described, for example, in French Pat. Nos. 1,399,100 and 1,379,984.

In summary, the shaft of the hydraulic motor follows exactly the rotary movements of the electrical stepping motor and hence precisely the commands embodied in the sequence of control pulses. This drive system has inherently a very slight time constant since the electrical portion operates at'relatively slight power and for that reason is well suitable for a rapid stroke sequence and thus also foe short abrupt movements in the forward or reverse direction. Since the drive virtually does not over-shoot or coast but stops immediately when the control pulses cease, the length of the waiting intervals during which the strand casting is no longer moved by the drive rollers is accurately adjustable.

The drive system according to the invention has the further advantage that the contraction of the strand during stand still can be easily equalized, namely by controlling the electrohydraulic stepping motor, first by a number of control pulses for forward operation, and thereafter by a smaller number of pulses for reverse rotation.

The power required for drawingthe: strand out of the molten mass may be temporarily very high. During such intervals of increased demand the pulling force exerted upon the strand should be as high as feasible. For that reason, the abovedescribed roller drive is preferably so designed that the nipping pressure of the rollers is made dependent upon the changes in torque occurring at the rollers to afford the assurance that the entire torque developed by the electrohydraulic stepping motor is transferred onto the strand driving rollers. If desired, the driving force furnished from the electrohydraulic motor to the rollers may also be automatically adapted to any changes in power requirements by providing a controllable transmission between the rollers and the hydraulic motor controlling the transmission in dependence upon changes in torque.

In the foregoing description it has been assumed that the mold or quantity of molten metal from which the strandshaped casting is being drawn, remains stationary and that only the driving rollers move intermittently. The invention, however, is not limited to this type of strand casting plant but is equally well applicable to plants in which the mold or melt container as well as the drive rollers must be driven. One way of applying the invention to such a plant is to provide two electrohydraulic stepping motors for the mold, on the one hand, and for the drive rollers, on the other hand. Preferably the two electrohydraulic motors are controlled from a single control monitor device which furnishes the programmed sequences of signals to the electrical portions of both electrohydraulic motors. As a result, the two drives are properly synchronized. For adjusting any desired synchronization ratio between the two drives, a frequency divider may be interposed between the common monitor device and one or both stepping motors. I

The invention will be further described with reference to the accompanying drawing, in which:

FIG. 1 shows schematically a strand casting plant with a drive system according to the invention by way of example;

FIG. 2 is a travel-time diagram explanatory of the method embodied in the system operation;

FIG. 3 is a schematic diagram of logic circuits embodied in the drive system of FIG. 1 for performing the operation represented in FIG. 2;

FIG. 4 is a block diagram of a modified drive system;

FIG. 5 is a block diagram of still another modification.

In the horizontal-type strand casting plant illustrated in FIG. 1, the metal strand 2 is drawn incrementally out of the mold or melt container 1 of the furnace by means of drive rollers 3. The rollers 3, or at least one of them, are driven by an electrohydraulic stepping motor 5. As explained, such a motor comprises an electrical stepping motor which responds to signal pulses, and a hydraulic motor which, by virtue of internal feedback, performs a follow-up travel as prescribed by the electric stepping motor. An electronic controller or monitor 4 supplies the electrical signal pulses to the electrohydraulic motor 5 in accordance with the desired technological requirements as represented, for example, in the form of punch cards, by a predetermined program.

The diagram in FIG. 2 represents the operating method of the strand casting plant, the travel distance of the strand casting being indicated along the abscissa. The ordinate indicates time. After a pulling interval a therefollows a waiting interval b. To take into account the contraction of the strand as it cools during the waiting interval, a relaxation interval 0 is provided and is followed by another waiting interval d. Thereafter the same cycle a to d is repeated. The length of the individual intervals can be choosen at will.

For performing this operation the monitor device 4 is designed as shown in FIG. 3. The signal pulses for controlling the electrohydraulic stepping motor to run in the forward or reverse direction are issued by a pulse transmitter 4a. The direction of the motor rotation depends upon which one of two AND gates 4bl and 4b2 is open during an interval of time determined by a timer 40 or 4d, each timer consisting, for example, of a monostable flip-flop. The running direction is under controlled by a bistable flip-flop 4f which receives setting and resetting pulses through logic circuitry 4i from an electronic counter 4g which counts the pulses received from an input reader 4h during the length of the timing signal received from the timer 40 or 4a through another AND gate 4e. The reader 4h responds to a program embodied in a punched card or strip as schematically indicated.

Once the reader 4h issues a starting pulse, the timer 4d passes pulses through the AND gate 4e to the counter 4g preadjusted by the reader 4h. The pulses simultaneously pass through the AND gate 4b1 to the pulse transmitter 4a which generates and issues the electrical pulse sequences for the electrohydraulic stepping motor 5. When the number precounted into counter 4g in the forward direction, corresponding to the travel a (FIG. 2), is attained, a command signal issues from the counter 4g through the stage 41' to the bistable flip-flop 4f so that the outgoing signal switches from the output a to the output c. Simultaneously the timing stage 4d is actuated and during the waiting interval b (FIG. 2) blocks the AND gates 4e, 4b1 and 4b2. Upon elapse of interval b the gates 462 and 4e are released, and pulses for the reverse direction are issued to the pulse transmitter 40 during the relaxation interval 0. When in the preadjusted counter 4g, the number of pulses corresponding to the programmed relaxation has been reached, an output pulse from counter 4g resets the flip-flop 4f and simultaneously reactivates the timer 4d. The timer then blocks the AND gates 4e, 4bl and 4b2 during the waiting interval d. Upon elapse of interval d, the AND gate 4b] is released so that the stepping motor 5 can again be controlled to move in the forward" direction.

In the modified system shown partially in FIG. 4 and otherwise corresponding to the one described above, a transmission 7 and a torque converter 8 are interposed between the drive rollers 3 and the electrohydraulic stepping motor 5. The torque converter 8 may be of electrical or mechanical type and also serves to measure the torque. When the torque converter 8 ascertains that the torque passes beyond a predetermined limit value, a feed back connection shown schematically at 9 switches the transmission 7 to a different transmission ratio. Simultaneously the valve 10 is actuated in dependence upon the torque measured in converter 8 and controls a hydraulic medium which correspondingly varies the pressure exerted between the drive rollers and the strand casting.

In the modified system according to FIG. 5, two electrohydraulic stepping motors 5 are provided for driving the rollers 3 on the one hand, and for shifting the mold or melt container of the casting plant on the other hand. A single electronic controller 4 monitors both stepping motors and supplies the individual motors 5 with pulses through respective frequency dividers 6 any desired speed or travel ratio between the two motors can be adjusted in a simple manner by correspondingly setting the two frequency dividers.

It should be understood that in a plant as described above with reference to FIG. 1 the electrohydraulic stepping motor may also be electrically controlled by manual operation; and it is also possible to provide for automation to a still further extent, namely in such a manner that the control device 4 is monitored directly from a computer which determines the required signals or signal sequences from operational data of the plant, such as temperature, pressure, stored quantity of molten metal, etc.

If desired, additional electronic circuitry may be provided for issuing a warning signal when the level of molten metal in the furnace becomes too low so that replenishment is needed. Electronic means may also be used for indicating the actual metal temperature in the furnace, for regulating the flame or other source of heat which keeps the molten metal sufficiently hot, or for regulating the temperature of the coolant for the metal mold or pool from which the strand casting is being drawn. The temperature of the strand itself may also be measured and indicated by electronic means.

We claim:

1. A drive system for drawing a continuous strand casting out of the mold of a continuous casting plant, comprising drive roller means engageable with the strand casting leaving the mold, a first electrohydraulic stepping motor in mechanical driving connection with said roller means for driving them in response to electrical signal pulses, a second electrohydraulic stepping motor for displacing the mold, and a pulse transmitting monitor electrically connected to said first and second stepping motors for programmed conjoint control of said first and second stepping motors by the same sequences of electrical pulses, said monitor comprising means for generating and supplying said electrohydraulic stepping motors with alternating and mutually time-spaced sequences of electrical signal pulses so as to draw the strand casting in incremental lengths of travel out of the mold and inserting an interval of rest between successive travel periods.

2. A drive system according to claim 1, comprising pulsefrequency dividers interposed between said monitor and said two electrohydraulic motors for maintaining a desired ratio of travel between said two motors.

3. A drive system for drawing a continuous strand casting out of the mold of a continuous casting plant, comprising drive roller means engageable with the strand casting leaving the mold, an electrohydraulic stepping motor in mechanical driving connection with said roller means for driving them in response to electrical signal pulses, and a pulse transmitting monitor electrically connected to said stepping motor for programmed control of said stepping motor, said monitor comprising means for generating and supplying said electrohydraulic stepping motor with alternating and mutually time-spaced sequences of electrical signal pulses and means for changing each second one of two pulse sequences to motor operation in the reverse direction and means for reducing the number of pulses in the second sequence relative to the number of pulses in the first sequence.

4. The method of drawing a continuous strand casting out of the mold of a continuous casting plant with the aid of drive rollers engaging the strand and an electrohydraulic stepping motor mechanically driving the rollers, which method comprises electrically controlling the electrohydraulic stepping motor by discrete and mutually time-spaced sequences of electrical pulses so as to draw the strand casting in incremental lengths of travel out of the mold and inserting an interval of rest between successive travel periods, including supplying to the electrohydraulic motor a number of sequential electrical pulses for forward rotation and thereafter a smaller number of pulses for reverse rotation so as to compensate for contraction of the strand.

5. The method of drawing a continuous strand casting out of the mold of a continuous casting plant with the aid of drive rollers engaging the strand and an electrohydraulic stepping motor mechanically driving the rollers, which method com prises electrically controlling the electrohydraulic stepping motor by discrete and mutually time-spaced sequences of electric pulses so as to draw the strand casting in incremental lengths of travel out of the mold and inserting an interval of rest between successive travel periods, separately displacing the mold by means of another electrohydraulic stepping motor, and conjointly controlling the two motors by the same sequences of electrical pulses.

6. The method of drawing a continuous strand casting out of the mold of a continuous casting plant with the aid of drive rollers engaging the strand and an electrohydraulic stepping motor mechanically driving the rollers, which method comprises electrically controlling the electrohydraulic stepping motor by discrete and mutually timespaced sequences of electric pulses so as to draw the strand casting in incremental lengths of travel out of the mold and inserting an interval of rest between successive travel periods, separately displacing the mold by means of another electrohydraulic stepping motor, conjointly controlling the two motors by the same sequences of electrical pulses and applying pulse-frequency division for maintaining a given ratio of travel between the two 5 motors. 

1. A drive system for drawing a continuous strand casting out of the mold of a continuous casting plant, comprising drive roller means engageable with the strand casting leaving the mold, a first electrohydraulic stepping motor in mechanical driving connection with said roller means for driving them in response to electrical signal pulses, a second electrohydraulic stepping motor for displacing the mold, and a pulse transmitting monitor electrically connected to said first and second stepping motors for programmed conjoint control of said first and second stepping motors by the same sequences of electrical pulses, said monitor comprising means for generating and supplying said electrohydraulic stepping motors with alternating and mutually time-spaced sequences of electrical signal pulses so as to draw the strand casting in incremental lengths of travel out of the mold and insErting an interval of rest between successive travel periods.
 2. A drive system according to claim 1, comprising pulse-frequency dividers interposed between said monitor and said two electrohydraulic motors for maintaining a desired ratio of travel between said two motors.
 3. A drive system for drawing a continuous strand casting out of the mold of a continuous casting plant, comprising drive roller means engageable with the strand casting leaving the mold, an electrohydraulic stepping motor in mechanical driving connection with said roller means for driving them in response to electrical signal pulses, and a pulse transmitting monitor electrically connected to said stepping motor for programmed control of said stepping motor, said monitor comprising means for generating and supplying said electrohydraulic stepping motor with alternating and mutually time-spaced sequences of electrical signal pulses and means for changing each second one of two pulse sequences to motor operation in the reverse direction and means for reducing the number of pulses in the second sequence relative to the number of pulses in the first sequence.
 4. The method of drawing a continuous strand casting out of the mold of a continuous casting plant with the aid of drive rollers engaging the strand and an electrohydraulic stepping motor mechanically driving the rollers, which method comprises electrically controlling the electrohydraulic stepping motor by discrete and mutually time-spaced sequences of electrical pulses so as to draw the strand casting in incremental lengths of travel out of the mold and inserting an interval of rest between successive travel periods, including supplying to the electrohydraulic motor a number of sequential electrical pulses for forward rotation and thereafter a smaller number of pulses for reverse rotation so as to compensate for contraction of the strand.
 5. The method of drawing a continuous strand casting out of the mold of a continuous casting plant with the aid of drive rollers engaging the strand and an electrohydraulic stepping motor mechanically driving the rollers, which method comprises electrically controlling the electrohydraulic stepping motor by discrete and mutually time-spaced sequences of electric pulses so as to draw the strand casting in incremental lengths of travel out of the mold and inserting an interval of rest between successive travel periods, separately displacing the mold by means of another electrohydraulic stepping motor, and conjointly controlling the two motors by the same sequences of electrical pulses.
 6. The method of drawing a continuous strand casting out of the mold of a continuous casting plant with the aid of drive rollers engaging the strand and an electrohydraulic stepping motor mechanically driving the rollers, which method comprises electrically controlling the electrohydraulic stepping motor by discrete and mutually time-spaced sequences of electric pulses so as to draw the strand casting in incremental lengths of travel out of the mold and inserting an interval of rest between successive travel periods, separately displacing the mold by means of another electrohydraulic stepping motor, conjointly controlling the two motors by the same sequences of electrical pulses and applying pulse-frequency division for maintaining a given ratio of travel between the two motors. 